The continuation monad parameterized by two answer types
We represent the the effectful expression e, whose type is
characterized by the judgement

Gamma; a |- e:tau; b

as a parameterized monad C a b tau. We represent an effectful function
type sigmaa -> taub of the calculus as an arrow type
sigma -> C a b tau.
Incidentally, this notational convention expresses the rule fun in AK07